Polyester-polyurethane hybrid resins are well-known in the art of thermoset molding compositions. These resins are normally tougher than polyesters and stronger, stiffer and less expensive than polyurethanes. Such hybrid resins usually comprise a hydroxy-terminated unsaturated polyester polyol, an ethylenically unsaturated monomer such as styrene and a polyisocyanate. They can be easily adapted to many common thermoset molding techniques employed in polyurethane and unsaturated polyester industries. Such hybrid resins are commercially available from Amoco Chemical Company under the trademark Xycon hybrid resins and are supplied as two component systems having an A and a B side. The A side contains the polyisocyanate and a polyester catalyst, while the B side contains the hydroxy-terminated unsaturated polyester polyol/styrene solution and optionally a polyurethane catalyst and/or filler.
Hybrid resins can be tailored to improve toughness and thermal properties of molded articles made from the resins. The balance between these two properties is dictated in part by the crosslink density within the hybrid. The greater the crosslink density, the higher the heat deflection temperature and glass transition temperature, i.e., thermal properties, but the lower the toughness. Toughness improvements can also be achieved by the addition of flexible moieties, such as ether groups into the polymer backbone, although at a sacrifice in thermal properties. It is desirable to improve the toughness of the hybrid without significantly reducing its thermal properties.
It is also desirable to reduce the shrinkage and improve the surface appearance of molded articles manufactured from hybrid resins. Shrinkage usually occurs during cure and can result in dimensional stability problems, such as warpage. Unacceptable surface appearances such as waviness or roughness may occur when hybrids are reinforced with fibers, such as glass, due to the fibers rising to the surface of the molded article. This phenomenon is commonly termed glass print-through.
It is further desirable to reduce the shrinkage of hybrid resins without having to add excess styrene. Excess styrene may be introduced when a low profile additive (LPA) is added to the hybrid to control shrinkage and improve dimensional stability and surface smoothness. An LPA is usually a thermoplastic polymer such as a vinyl acetate polymer, acrylic polymer, polyurethane polymer, polystyrene, butadiene styrene copolymer, saturated polyester and polycaprolactone. These are typically non-reactive polymers (non-reactive end groups) of high molecular weights (10,000 to 200,000), which are supplied in a vinyl monomer such as styrene to reduce the viscosity of the thermoplastic to a workable range. The LPA tends to phase separate from the polyester during cure, resulting in thermoplastic domains that induce stresses within the system. These stresses result in microcracks and microvoids in molded products containing the LPA. These internal imperfections reduce the amount of shrinkage that occurs during cure, but have the drawback of weakening the polyester matrix. Another drawback associated with the use of an LPA is the quantity of styrene required to achieve a sufficient amount of thermoplastic to produce a low profile effect in the thermoset. Typical LPA-modified polyesters contain up to about 50-60 weight percent styrene monomer. When there is more than 1 mole of styrene per equivalent of unsaturation in the polyester, the product has a tendency to have high shrinkages. However, these shrinkages are somewhat counterbalanced by the micro-imperfections developed during the phase separation of the thermoplastic from the thermoset.
The prior art, such as U.S. Pat. No. 4,822,849, teaches reducing the shrinkage of hybrid resins by reducing both the styrene level and unsaturation level within the hybrid. Lower shrinkage is achieved by reducing the crosslink density, but, as described above, this may lead to reduced thermal properties of the hybrid resin. The prior art, including U.S. Pat. No. 4,280,979, also describes the preparation of unsaturated polyester diols which can be reacted with a polyisocyanate and a polymerizable ethylenically unsaturated monomer to produce polyurethane/vinyl copolymers. Both patents are incorporated herein by reference.
Also the prior art, such as U.S. Pat. Nos. 4,310,448, 4,305,857, 4,093,569, 4,310,449, 4,147,680 and 4,305,858, teaches preparation of dispersions of polyisocyanate polyaddition products in compounds having hydroxyl groups. These patents teach the reaction of an organic polyisocyanate and compounds having primary and/or secondary amino groups and/or hydroxyl groups and/or ammonia in compounds having at least one hydroxyl group. All of these patents teach carrying out the reaction in the presence of more than 2 percent by weight of water, a polymer latex or in a solution of an ionic polyurethane. These patents further teach that it is preferred to react approximately equivalent quantities of isocyanates and amino functional compounds. It is stated that a limited excess of isocyanate may also be used, but the products then obtained have a high viscosity due to the isocyanate reacting with the dispersing agent, which apparently is undesirable for the described polyaddition products. The present invention teaches the preparation of two phase molding compositions which are prepared in a substantially water-free polyol solution with an excess of isocyanate (between about 3:1 and about 100:1). It is undesirable in the present invention to use water because water will react with the isocyanate resulting in an undesirable foam product. It has further been found that the use of an excess of isocyanate has unexpected advantages of improved shrinkage, surface and impact properties in the disclosed molding compositions.
For the purpose of this disclosure, the term "cure" or "curing" means the transformation of the hybrid resin system from a liquid to a gel or solid state. This curing occurs at the time of molding by cross-linking of the reactive sites in the hybrid system, including the reaction of the isocyanate with active hydrogen-containing compounds. Further, the curing of the hybrid resin system occurs via the vinyl addition reaction between the ethylenically unsaturated monomer and the unsaturated polyester polyol. Depending on the catalyst employed, curing can optimally occur at the time of molding at temperatures of about 25.degree. C. to about 150.degree. C. for a time of about 10 mins to about 24 hrs.
As used herein, the term "stoichiometric index" refers to the value obtained by dividing the number of active NCO groups of the isocyanate compound by the sum of the active NH plus OH groups of the nitrogen-containing compound and the polyester polyol of the invention, i.e., NCO:(NH+OH). The term "substantially water-free", as used herein, means a water content of less than 1 percent, preferably less than 0.2 percent and most preferably less than 0.05 percent. The term "polyfunctional", as used herein, is intended to include functionalities of two or greater.
It is an object of the present invention to provide novel hybrid resin molding compositions having a first, soft polyurea phase within a second, more rigid polyester-polyurethane phase. It is another object of this invention to provide a hybrid resin that offers improved shrinkage control. It is yet another object to provide an improved molding composition which has improved impact strength without significantly sacrificing thermal properties, resulting in a novel balance of thermal/impact properties. It is a further object to provide a polyester-polyurethane hybrid resin molding composition which offers improved surface properties in molded articles manufactured therefrom and at a faster gel time.
To accomplish the above objects, the invention provides novel two phase molding compositions and processes for making improved molding compositions. In yet another aspect, the invention enables hybrid resin systems having an A and a B side, which, when reacted, provides improved two phase molded products.